- #1
mishima
- 570
- 36
Homework Statement
I am looking for the base, collector, and emitter voltages in the following circuit:
Homework Equations
KCL
KVL
offset voltage = VB - VE
saturation voltage = VC - VE
The Attempt at a Solution
First I made a matrix without assuming any values for offset or saturation voltages. Doing so gave me this:
1 55.56 14.7 | 678.72 KCL with transistor as node
9/500 1 9/34 | 12.216 KVL right hand mesh
17/250 34/9 1 | 46.149 KVL left hand mesh
The determinant for this matrix is 0, and seems to have infinite solutions. So that was not a good approach here. I checked the circuit with CircuitLab, and sure enough the voltages work with my matrix. Its just that the matrix can't produce them unfortunately... (10.07, 9.539, 9.428 for Vbase, Vcollector, and Vemitter). First, I was wondering if this always happens with a saturated bipolar transistor. Or, this isn't a textbook problem, is that the only reason the matrix didn't work out?
A much easier solution was to just assume that the offset voltage was 0.7 V and saturation voltage (Vce) was 0.2 V. Those 2 equations
Vbase - Vemitter = 0.7
Vcollector - Vemitter = 0.2
along with the first equation in the matrix gave solutions consistent with CircuitLab to within a hundreth of a volt. So second, I'm just kind of wondering what makes the first assumption valid. I was told offset voltage is not a constant. The Vce assumption makes sense just looking at a datasheet for the 2n2222. But why can I say that Vbe is 0.7 with confidence here?
Thanks.